2017MNRAS.472.4953L

We show the peak magnitude for orphan afterglows from the jets of gravitational wave (GW) detected black hole/neutron star - neutron star (BH/NS-NS) mergers highly depend on the jet half-opening angle θ_j. Short γ-ray bursts (GRBs) with a homogeneous jet structure and θ_j > 10°, the orphan afterglow viewed at the typical inclination for a GW detected event, 38°, are brighter at optical frequencies than the comparable macronova emission. Structured jets, where the energetics and Lorentz factor Γ vary with angle from the central axis, may have low-Γ components where the prompt emission is suppressed; GW electromagnetic (EM) counterparts may reveal a population of failed-GRB orphan afterglows. Using a Monte Carlo method assuming an NS-NS detection limit we show the fraction of GW-EM counterparts from homogeneous, two-component, power-law structured and Gaussian jets where the variable structure models include a wide low energy and Γ component: for homogeneous jets, with a θ_j = 6° and typical short GRB parameters, we find r-band magnitude m_r <= 21 counterparts for ∼13.6 per cent of GW detected mergers; where jet structure extends to a half-opening angle of 25°, two-component jets produce m_r <= 21 counterparts in ∼30 per cent of GW detected mergers,power-law structured-jets result in ∼37 per centand Gaussian jets with our parameters ∼13 per cent. We show the features in the light curves from orphan afterglows can be used to indicate the presence of extended structure.